Field of the Invention
The present invention relates to an automatic transmission for a vehicle, and more particularly, to a planetary gear train of an automatic transmission for a vehicle which is capable of implementing at least nine forward speeds using a minimum number of configurations, improving power transmission performance and fuel efficiency by increasing a span of a gear shift ratio, and ensuring uniformity of an inter-stage ratio between neighboring gear shift stages.
Description of Related Art
The recent increase in oil prices causes carmakers to meet global demands of improving fuel efficiency.
Accordingly, researches are being conducted on engines in terms of reducing weight and improving fuel efficiency by down-sizing, and researches are also being conducted to ensure both drivability and competitiveness by maximizing fuel efficiency by implementing an automatic transmission with multiple stages.
However, in the case of the automatic transmission, the number of internal components is increased as the number of gear shift stages is increased, which may cause deterioration in terms of mountability, production costs, weight, and power transmission efficiency.
Therefore, in order to increase an effect of improving fuel efficiency by implementing an automatic transmission with multiple stages, it is important to develop a planetary gear train capable of maximizing efficiency using a small number of components.
In this respect, recently, an eight-speed automatic transmission has been implemented, and research and development is being actively conducted on a planetary gear train that may implement gear shift stages for eight or more speeds.
However, in the case of the recent eight-speed automatic transmission, a span of a gear shift ratio is maintained at a level of 6.5 to 7.5, and as a result, there is a problem in that the recent eight-speed automatic transmission has no great effect of improving fuel efficiency.
Further, because it is impossible to ensure linearity of inter-stage ratios between neighboring gear shift stages in a case in which a span of a gear shift ratio in the case of the eight-speed automatic transmission is increased to 9.0 or more, driving efficiency of the engine and drivability of the vehicle deteriorate.
Accordingly, there is a need for development of a highly efficient automatic transmission with the gear shift stages for nine or more speeds.
The information disclosed in this Background of the Invention section is only for enhancement of understanding of the general background of the invention and should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art already known to a person skilled in the art.